1. Field of the Invention
The present invention relates to a ball screw device and a linear motion device which do not induce decreases in load capacity and in rigidity with a restraint of reduction in the number of load balls even when spacers are disposed between the load balls, which enhance a circulative characteristic of the spacers by minimizing friction between the load balls and the spacers, and which prevent a deterioration of operability, an occurrence of noises and deteriorated quality of a sound produced, and frictional damage to the balls.
2. Related Background Art
In the ball screw device, as shown in FIG. 33, helical screw grooves 3, 4 corresponding to each other are formed in an outer peripheral surface of a screw shaft 1 and in an inner peripheral surface of a nut 2. A multiplicity of balls 5 are so disposed as to be capable of rolling in a helical circulation path defined by the two screw grooves 3, 4. When one of the screw shaft 1 and the nut 2 is moved in the axial direction by relatively rotating the screw shaft 1 and the nut 2, the screw shaft 1 and the nut 2 make smooth helical motions through rolling of the multiplicity of balls 5.
In the thus constructed ball screw device, the balls 5 are densely disposed within the screw grooves 3, 4 and roll in the same direction in the individual screw grooves 3, 4. On this occasion, however, at a contact point between the balls adjacent to each other, the balls 5 rolling in the directions opposite to each other come into contact with each other enough to mutually hinder the rolling thereof. As a result, there might arise a variety of problems in which free rolling of the balls 5 is thus hindered, an operability of the balls 5 is deteriorated, frictional damage to the balls 5 is induced, a torque fluctuates, and noises increase.
To cope with these problems, Japanese Patent Application Laid-Open Publication No. 56-116951 discloses a construction wherein an elastic member for making the balls spaced away from each other is disposed between the balls receiving a load, and an annular member making a circulative movement together with the balls is loosely fitted to the outside of the elastic member. Japanese Patent Application Laid-Open Publication No. 57-101158 discloses such a construction that a shim is retained between the adjacent balls and serves to prevent a rolling friction between the balls.
Further, Japanese Utility Model Application Laid-Open Publication No. 1-113657 discloses a construction in which a spacer ball 6 formed from a resin is, as illustrated in FIG. 34, interposed between the balls 5 receiving the load, thereby preventing the impingement of the balls on each other and restraining an occurrence of noises.
Incidentally, what is similar to the ball screw device described above may be exemplified by a linear guide constructed of a guide rail extending in the axial direction, a slider provided astride of this guide rail, and balls serving as rolling members and interposed between the guide rail and the slider. The above-mentioned ball screw device and linear guide are generically referred to as a linear motion device in the present specification. The linear motion device is defined as being constructed of an outer member, an inner member facing to this outer member through a gap, a multiplicity of balls disposed between the outer and inner members, and spacers interposed between those balls.
For example, in the case of the linear guide, the slider having substantially U-shape section is mounted astride of the guide rail having an angular bar-like shape, track grooves are formed respectively in an outer surface of the guide rail and in an inner surface of the slider which faces thereto, and the multiplicity of balls as the rolling members are loaded in the track grooves, whereby the slider and the guide rail make relative linear motions with the aid of the rolling members circulated while rolling. In the case of this type of linear guide, the slider is defined as the outer member, while the guide rail is defined as the inner member. On the other hand, another type of linear guide has such a construction that an angular slider is accommodated in a recessed portion of the guide rail taking substantially the U-shape in section, and the balls are loaded in the track grooves formed respectively in the inner surface of the guide rail and in the outer surface of the slider which faces thereto. In this case, the guide member is defined as the outer member, while the slider is inner member.
Further, in the ball screw device, as described above, the screw shaft, of which the outer surface is formed with the helical screw groove, is inserted into the nut with its inner surface formed with the helical screw groove, and the multiplicity of balls are loaded in the two screw grooves facing to each other. With these balls making the rolling circulation, the nut and the screw shaft perform their relative rotational and linear motions. Accordingly, in the case of the ball screw device, the nut is defined as the outer member, while the screw shaft is defined as the inner member.
To summarize, the outer member of the linear motion device indicates the slider or the guide rail in the case of the linear guide, and indicates the nut in the case of the ball screw device. The inner member indicates the guide rail or the slider in the case of the linear guide, and indicates the screw shaft in the case of the ball screw device.
An example of the above linear motion device using a spacer is disclosed in Japanese Patent Application Laid-Open Publication No. 5-126148, wherein as shown in FIG. 35 a spacer 7 having two concave surfaces 6, 6 contiguous respectively to balls 5, 5 is disposed between the balls 5, 5 adjacent to each other. Further, as for a bearing, Japanese Patent Application Laid-Open Publication No. 62-118116 discloses a structure that as shown in FIG. 36 a hollowed pipe-like spacer 8 is disposed between the adjacent balls 5, 5. The spacer 8 is formed by cutting off to a predetermined dimension a steel pipe of which a diameter is smaller than a diameter of the ball 5. Further, as disclosed in Japanese Patent Application Post-Exam Publication No. 40-24405, a partition member disposed between the adjacent balls has two spherical concave portions each facing to a ball, a radius of which is slightly larger than a radius of the ball. A through-hole formed at the center of the spherical concave portion of the partition member is used as a reservoir of lubricating oil.
A problem inherent in only the ball screw device described above is that the spacer, such as the elastic member, the annular member and the shim etc, is provided in each of the ball screw devices disclosed in Japanese Patent Application Laid-Open Publication Nos. 56-116951 and 57-101158, and therefore the number of the balls receiving the load is reduced, with the result that a load capacity and a rigidity of the ball screw device decrease.
Additionally, the spacer, such as the elastic member, the annular member and the shim etc, induces an impingement upon the screw groove enough to cause a skew (from a proper posture) of the spacer, resulting in a decline of a circulative characteristic of the spacer.
In the ball screw device disclosed in Japanese Utility Model Application Laid-Open Publication No. 1-113657, as shown in FIG. 34, the number of the balls receiving the load is, e.g., 10, while the number of the spacers 6 is, e.g., 10, whereby a spacing between the balls 5 receiving the load becomes large, the number of the balls 5 receiving the load is approximately halved, and both of the load capacity and the rigidity of the ball screw device decrease.
Another problem with respect to the linear motion device in the prior art explained above is that it is desirable to make a slide friction between the spacer and the ball as small as possible in terms of considering an operability of the linear motion device. However, as shown in FIG. 35, if a curvature (1/r) of the ball 5 is equalized to a curvature (1/R) of the spacer concave surface 6, sliding occurs when the ball comes into contact with the entire concave surface of the spacer, with the result that the frictional force increases and the operability is deteriorated.
It is very important in this linear motion device to control a thickness of the spacer in order to set an optimum total gap in each train of balls endlessly circulated, i.e., to control an inter-ball span when the spacer is interposed therebetween. But when manufacturing the spacer 7 aiming at forming the concave surface 6 having the same curvature (1/R) as the curvature (1/r) of the ball 5, there might be formed the concave surfaces 6 having larger and smaller curvatures than the curvature (1/r) of the ball 5 because of a dimensional scatter. Especially if the curvature (1/R) of the concave surface 6 of the spacer 7 is smaller than the curvature (1/r) of the ball 5, the balls are destabilized when the spacer 7 is disposed between the balls 5, and it is extremely difficult to measure a dimension between the balls 5 (which is a thickness of the spacer 7). The problem is therefore that the spacer 7 exhibiting a high accuracy can not be manufactured. Moreover, in a structure as shown in FIG. 36, it is required that the diameter of the spacer be smaller than the diameter of the ball. However, as shown in FIG. 36, in the case of the pipe-like spacer 8, a minor diameter of the pipe-like spacer 8 becomes small due to the thickness thereof, and the balls 5 are hard to stabilize. There is no alternative but to increase the major diameter of the pipe-like spacer 8 for stabilizing the balls 5. Consequently, there arises a problem in which the spacer 8 interferes with other components during the circulation.
According to Japanese Patent Application Post-Exam Publication No. 40-24405, the through-hole formed in the partition member is used as the reservoir of the lubricating oil for preventing a seizure if a rotating velocity and a revolution velocity of the ball are high as in the case of a rolling bearing. In the linear motion device, however, almost no seizure problem arises because of the above velocities being by far lower than those of the rolling bearing. A further problem in the prior art example is that a lubricating oil reserving capacity of the through-hole is insufficient.